The present invention relates to an ink jet recording apparatus for utilizing air streams to eject ink to record characters, images, or the like on a recording medium.
One prior ink jet recording head utilizing air streams is illustrated in FIGS. 1 through 4. As shown in FIGS. 1 and 2, the ink jet recording head, designated by the reference numeral 112, includes a body 101, an air nozzle plate 102 of an insulating material attached to the outer end of an outer wall 101a of the body 101, and an ink nozzle plate 103 of an insulating material attached to the outer end of an inner wall 101b of the body 101. The outer and inner walls 101a, 101b of the body 101 define therebetween an air chamber 104 communicating with an air passage 105 defined between the air nozzle plate 102 and the ink nozzle plate 103. The body 101 has an ink chamber 106 defined inwardly of and by the ink nozzle plate 103 and the inner wall 101b. The ink nozzle plate 103 has a plurality of ink discharge ports 107, and the air nozzle plate 101 has a plurality of air discharge ports 108 in alignment with the ink discharge ports 107, respectively. A common electrode 109 of flat configuration is mounted on the outer surface of the air nozzle plate 102 around the air discharge ports 108. Separate control electrodes 110 are mounted on the inner surface of the ink nozzle plate 103 around the ink discharge ports 107. The electrodes 110 are connnected to signal sources 111. An air supply passage 113 has an end connected to the air chamber 104, and an ink supply passage 114 has an end connected to the ink chamber 106.
FIG. 3 shows an overall arrangement of an ink jet recording apparatus incorporating the ink jet recording head 112. In FIG. 3, the other end of the air supply passage 113 is coupled to an air source 115, and the other end of the ink supply passage 114 is coupled to an ink tank 116. An air supply passage 117 branched from the air supply passage 113 is connected to an upper end of the ink tank 116.
Operation of the conventional ink jet recording head will be described below. Air is supplied from the air source 115 via the air supply passage 113 into the air chamber 104, from which it flows through a sharp bend as an air layer into the air passage 105 at a constant speed. The air flow is sharply bent in the vicinity of the air discharge ports 108 and the ink discharge ports 107 and goes into the air discharge ports 108. The ink chamber 106 is always filled with ink fed from the ink tank 116 through the ink supply passage 114. The ink in the ink tank 116 and the ink chamber 106 is subjected to a constant pressure by the pressure of air supplied from the air source 115 via the air supply passage 117 into the ink tank 116. The pressure of the ink in the ink discharge ports 107 is substantially equalized under the constant pressure to the pressure of air flowing near the ink discharge ports 107 while the ink jet recording head is not in operation, so that the meniscus of the ink in the ink discharge ports 107 is held at rest. When there is a potential difference between the common electrode 109 and the control electrodes 110, the meniscus of the ink in the ink discharge ports 107 is stretched toward the air discharge ports 108 by an electrostatic force produced by the potential difference. Since there is an abrupt change in the pressure gradient created by the air flow in the air passage 105 from the ink discharge ports 107 to the air discharge ports 108, when the meniscus of the ink in the ink discharge ports 107 is stretched beyond a certain length or interval, the ink is abruptly accelerated from the ink discharge ports 107 across the air passage 105 toward the air discharge ports 106 and expelled out of the air discharge ports 106.
FIG. 4 schematically shows the arrangement of FIG. 2 as a simplified system. Conditions for stably holding the ink in the ink discharge ports 107 will be described with reference to FIG. 4. The pressure Pa of air delivered into the air chamber 104 is substantially equal to the air pressure in the air source 115 if the air supply passage 113 from the air source 115 to the head 112 has no pressure loss or only a negligible pressure loss. The pressure Pi of ink in the ink chamber 106 is substantially equal to the ink pressure in the ink tank 116 and also the air pressure in the air source 115. Therefore, assuming that the air supply passages 113, 117 have no pressure loss or only a negligible pressure loss, the air pressure Pa is approximately equal to Pi. In order to keep a meniscus 118 of ink in the ink discharge port 107, the ink pressure Pi in the ink discharge port 117 is required to be substantially equal to the air pressure Pn in the vicinity of the ink discharge port 107. Therefore, the conventional ink jet recording head has been dimensionally and structurally designed to reduce the pressure loss of the air in the air passage 105 between the air and ink nozzle plates 102, 103 so that the air pressure Pa is approximately equal to the air pressure Pn to make the air and ink pressures Pa, Pi approximately equal to each other, thus stably keeping the meniscus 118 at the ink discharge port 107.
With the multinozzle ink jet head 112 having the plural air and ink discharge ports 108, 107, it is quite difficult to equalize and stably keep the meniscus at the respective ink discharge ports 107. The air passage 105 has a small thickness of about 100 micrometers, and a slight variation in the thickness of the air passage 105 would result in a change in the air pressure Pn near the ink discharge port 107. It has been highly difficult to manufacture the ink jet recording head while controlling the thickness of the air passage 105 to be uniform at all of the ink discharge ports 107.
One conventional solution has been to place a spacer of constant thickness between the air and ink nozzle plates 102, 103, thus spacing them from each other by a constant distance.
Where such a spacer is bonded by adhesive layers between the air and ink nozzle plates 102, 103, the thickness of the adhesive layers used tends to differ from place to place, and the air passage 105 is often apt to have varying thicknesses. As a result, the air pressure Pn near the ink discharge ports 107 varies due to thickness irregularities of the air passage 105. The ink meniscuses are therefore not, uniformly stabilized, and the responses, the amounts of discharged ink, and the threshold voltages (minimum recording voltage) are varied from discharge port to discharge port, resulting in different recording characteristics exhibited by the ink jet recording head.
When the ink jet recording head is shocked to cause ink to flow from the ink discharge port 107 into the air passage 105, such ink is trapped and cannot easily be removed irrespective of the air flow in the air passage 105. This is because air is liable to flow out more easily from an adjacent air discharge port 108 than from the air discharge port 108 where the ink is trapped. The recording characteristic at such a disabled head area is lowered, and hence the multinozzle ink jet recording head has varying recording characteristics.